Eve, Adam and how we almost didn’t make it

We’re incredibly lucky. The world could have been a completely different place; a world without humans. There would have been no Giza pyramids, no wall of China, no Roman empire, no International Space Station. *dramatic pause* No hummus.

It’s a story of hardship, disasters and conflicting theories.

In a world far far away, a long time ago

The vast and arid Sahara desert up till only a few thousand years ago.

Modern humans show a very limited genetic variation. In fact, if we randomly take two people from anywhere in the world and compare their DNA, they would be more genetically similar than two mountain gorillas from the same troop*. All humans alive today are, in essence, cousins.** But how can that be? What happened to our genetic variation? To get the answer to that we need to – once again – visit those prehistoric grassy plains in Africa from where we came.

Several hundred thousands years ago, modern humans existed only in Africa, and were slowly expanding north through what was then the fertile grassy plains of Sahara. But then, some 70,000 years ago, the Indonesian volcano Toba erupted in a vast cloud of ash and smoke, triggering a volcanic winter of several years and kick-starting the latest ice age. Our population suddenly collapsed to a fraction of its previous size and only a few thousand humans remained in the whole world.

What the Toba super-eruption could have looked like.

This was obviously disastrous. Our genetic gene pool shrunk to a puddle and even a single epidemic outbreak could easily had killed off all humans in one go. And thus, even to this day, human beings have a very limited genetic variation.

It’s a lovely theory, full of drama and perseverance against all odds. But unfortunately it looks like it might be incorrect. The Toba super-eruption did take place, and the latest ice age (or rather the latest glacial period in the current ice age) did start about that time, but studies of the full genetic material of humans show no evidence of a drastic population bottleneck at that time. Bummer.

There is however another theory that can explain our lack of genetic variation.

Going out with a fizz, not a bang

I don’t know why this Homo ergaster man looks so ashamed. After all, he’s one of ancestors of modern human beings. *pause* Oh. I get it now.

In the year 2000, a study of human population bottlenecks found that there had indeed been a drastic reduction of the human population. But it hadn’t happened 70,000 years ago. And it wasn’t a single dramatic event. Instead, the study found that early humans most likely suffered a sustained drawn-out population bottleneck effect ca. 2 million years ago. It seems likely that our world-wide population was as low as 2,000 individuals for perhaps as long as 100,000 years.

Now, 2 million years ago human beings weren’t modern. In fact, even though they were our direct ancestors and begun the unbroken lineage to the current human population, they were sufficiently different from us to be defined as a different species: Homo ergaster.

But regardless what chronospecies we belonged to, it seemed like we had a really hard time. With a world population of only 2,000 individuals for an extended period of time, we would – in todays conservation terminology – be classified as a critically endangered species. But, as luck would have it, instead of going extinct, we spawned several new species from isolated pockets of populations. And even if most of those species went extinct, more than a million years later Homo ergaster was still around, together with a sister species that had spread to Asia: Homo erectus. And Homo heidelbergensis had appeared (and would later give rise to Homo neanderthalensis, the elusive and still un-named Denisovans as well as modern humans: Homo sapiens). So our toughest challenge was also the key to our most proliferate speciation and helped us to spread across the world.

Mitochondrial Eve

What Eve actually looked like. Probably. Well, possibly anyway.

But a story about the origin of modern humans wouldn’t be complete without mentioning Adam and Eve. No, not the biblical ones. The real ones. Let’s start with Eve, since she came first.

All humans in the world today are descendants of a single woman who lived some 200,000 years ago. She is known as Eve, or Mitochondrial Eve.

Now, the concept of Mitochondrial Eve might warrant some further explanation. Mitochondria are the power plants of our cells, converting glucose and other molecules to ATP – the main chemical energy form for the cell. They have their own DNA, separate from the main DNA that is housed in the cell’s nucleus. It stays unaffected by cell division and recombination and only really change through the slow process of mutation. As mitochondria are inherited exclusively from the mother (sperm cells only transfer nuclear DNA to the egg cell during fertilisation), we can trace the human lineage on the maternal side by studying mutations in the mitochondrial DNA.

Even though all living human beings are descendants of a single female, it doesn’t follow that there was only one woman left, and that her children had to mate with each other (ew!). Rather, many thousand women probably existed, spread over a number of tribes. But chance would have it that all the other women’s daughters sooner or later didn’t have any daughters of their own. And so their lineages of mitochondrial DNA were broken.

The end result is that all humans alive today have inherited their mitochondria from the set that an ordinary tribes woman somewhere in eastern Africa carried in her cells some 200,000 years ago. Which I think is pretty cool.

Y-chromosomal Adam

There is also a real Adam, called Y-chromosomal Adam, even though he’s not as interesting as Eve.

Y-chromosomal Adam is the man whose Y-chromosome can be traced to every living man today. Since the Y-chromosome doesn’t recombine with other Y-chromosomes (only males carry one, and they only carry a single one), it can be tracked back in time like the mitochondria of Eve. But, since only men carry Y-chromosomes, only men are direct descendants of Adam, not all humans. Which is why it’s not as interesting as the mitochondrial Eve thing.

Don’t know what I dislike most. The fact that the serpent is supposed to be the bad guy, or that the woman is portrayed as being so devious. Or that God is being such a big baby about the whole situation.

A bit more interesting is that Eve lived about 200,000 years ago and Adam some 142,000 years ago. That’s a gap of 60,000 years – give or take a few thousand years. So Adam and Eve never met. In fact, the biblical story sort of sounds like a crude misinterpretation of our phylogenetic history, as if it had been told to some illiterate goat herders many thousand years ago, passed on as a magic fairytale from generation to generation until finally written down thousands of years later. Which of course it couldn’t have been. I mean, who would have told them the story for a start? The human technology at the time – although impressive – did not include advanced genetic sequencing and supercomputers for analysing the results.

So it looks like the biblical Adam and Eve have very little to do with reality. Something we probably should be thankful for, considering the story’s inherent sexist, demeaning and generally disturbing nature. It is a fairytale, and not even a very good one.

Luckily the truth is much more interesting.

* Mountain gorillas live in groups of 20-50 animals called troops. One dominant male (a silver back) mates with all the females in the troop, limiting the genetic variation within the group. But even so, they display a much higher level of genetic variation than humans.

** Our limited genetic variation might in part help explain our instinctive dislike of incest. Since we’re all so very closely related already, any incestuous behaviour might result in heavily inbred offspring, with a high probability of them ending up suffering from some horrible inherited disease. This is not so much an issue in other, more genetically diverse, species. Like rats, for instance, who are much more liberal in their mating behaviour.

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I think this is fascinating. And I think if people thought about this – and really internalized it – we wouldn’t fight with each other as much. Because we’re all related, you know? But people don’t want to think about that. It’s too scary to be related to the people you hate (for no good reason.)

Indeed. The truth is scary and difficult to handle. And to think that normal intelligent people can believe that there’s such a big difference between people just because of their appearance, when we’re more or less identical. It’s sad and tragic, is what it is. The differences we might preview is obviously just cultural and other learned behaviour. And as such very flexible. Compromises should be easy to reach.

I guess the point was to show that women are not only devious but also gullible? Although that seems an unlikely combination. But that’s what happens if you leave goat herders in charge of your religious texts. Goat herders are notoriously misogynistic.

This got me to thinking how hard it was just to survive not too far back into the past. I wonder how we got to the place in life where I am worried I won’t survive because my iPhone battery goes dead so quickly.

Fascinating post, as always, Andreas.
I find it a cruel irony that though our species (ancestrally, at least) was reduced to roughly a thousand members for many millennia, we turn a blind eye to species rapidly becoming endangered or going extinct. I think the mountain gorillas might have a few words for us. (In fact, if you’ve read Ishmael, by Daniel Quinn, they do; and a few things to say about myths told by goat-herders, to boot.)

Well, we’re not completely turning a blind eye to it, but, as a species, we don’t have a coherent body of government to make global decisions. We’re just too fragmented and territorial.

Also, it’s not about saving single species (like the mountain gorilla) but more about saving ecosystems, something I mentioned in Species fixation. And that takes even more effort and cooperation between disparate groups of people.

I think the fact that H. sapiens alone didn’t become extinct during the last major glaciations (or soon thereafter) was only a temporary reprieve. I also doubt it was because we had a decisive edge in any way over the others: chance seems to have played a large part. And now, the behaviours that all human variants probably shared, and which doubtless helped the human family survive as long as it did – exploitation of environments to the point of destruction and innate competition between groups – will lead to our destruction. First it’ll wreck our civilisation: and then, when we’re back to isolated hunter-gatherer bands trying to eke out a life in a world environment that’s turned hostile through our civilisation ruining it, we’ll also be vulnerable to extinction again. And will probably succumb.

Yes, random chance most likely played a big role in the evolution and survival of our species, as is often the case in biology. And it is true that several of the other species of humans also shared the urge to explore and invent.

But it does seem that something specific happened to our own species some 70 000 years ago, that put our imagination and inventiveness into hyperdrive. It’s called the cognitive revolution, where improvements in our cultural expressions and tool-making suddenly gained break-neck speed. It’s believed to result from some form of mutation governing the structure of our brains. With this, we suddenly had the upper hand and rapidly spread across the world, extinguishing (or assimilating) other species as we came across them.

So yes: we’re still very much in the hands of random luck, but we are also equipped with more imagination and ability to innovate than any other species before us. So what’s going to happen? Who knows. The odds aren’t greatly in our favour, but we do seem to have an uncanny knack for getting out of tricky situations.